The invention relates to a method for producing a component having a substrate made of SiC, which comprises at least one ohmic contact and at least one Schottky contact. The invention relates, in particular, to a method in whichxe2x80x94possibly repeatedlyxe2x80x94temperatures in excess of about 950xc2x0 C. are reached.
In order to actually achieve the theoretically very low on-state losses of SiC components such as Schottky and pn diodes or else FETs in real structures, it is necessary for ohmic contacts to be available whose contact resistance is so low that it is negligible relative to the internal resistance of the component. A value of below 10xe2x88x925 xcexa9 cm2 is generally sought for this contact resistance. Furthermore, this contact must be stable, i.e. its electrical properties, for example, must not be impaired in the event of exposure to a temperature of up to 300xc2x0 C.
These requirements have been met heretofore only with ohmic contacts which are produced as alloyed Ni contacts on n-doped SiC. In that case, the alloying of the contact has to be carried out at temperatures of at least 950xc2x0 C. Other metals such as, for example, Ti can, with specific surface preparations, also yield an ohmic contact with sufficiently low resistance directly after the deposition, but subsequent thermal loading leads to unacceptable irreversible impairment of the ohmic contact at a temperature as low as 150xc2x0 C.: after brief heating to 300xc2x0 C., the ohmic contact already exhibits Schottky behavior. Thus, the contact using Ti does not represent a viable alternative to the conventional Ni contact.
One example of the difficulty in coordinating the contact production and other process steps with one another in such a way that they do not adversely affect one another is the process for producing Schottky diodes: it is endeavored to apply the Schottky metal (Ti or else Ni) to the SiC surface by sputtering or vapor deposition directly after a high-temperature annealing step at more than 1400xc2x0 C. under a hydrogen atmosphere. After the annealing process, the surface is in a state which is highly suitable for the production of the Schottky contact. However, if an ohmic contact is subsequently produced on the rear side of the wafer, which contact must be subjected to heat treatment at 950xc2x0 C. as described above, then the Schottky metallization layer on the front side has lost its rectifying behavior as a result of the heat treatment. Therefore, the current procedure is as follows: after the hydrogen annealing, firstly the rear-side contact is produced and then e.g. wet-chemical steps are carried out in an attempt to condition the front side in such a way that it is suitable for the Schottky metallization. In that case, the reproducibility and the rectifying behavior are generally distinctly poorer than directly after the heat treatment in a hydrogen atmosphere.
Thus, as a result of the necessary annealing step for alloying Ni at 950xc2x0 C., significant limitations arise with regard to the sequence of the overall process in the production of the component.
The Patent Abstract pertaining to Japanese patent application JP 58-138027 discloses the general production of an ohmic Ni contact on an SiC substrate by vaporization of metal and subsequent heating of the substrate. However, there is no indication in respect of the order of the method steps and, in particular, in respect of the position of the step in which metal is deposited on the substrate, in a method for producing components having an ohmic contact and having a Schottky contact.
German published patent application DE 20 28 076 A specifies a method with which a reliable metallic contact is produced on an SiC semiconductor at a comparatively low temperature of e.g. 700xc2x0 C. The position of the step in which metal is deposited on the semiconductor is once again not revealed in DE 20 28 076 A.
U.S. Pat. No. 5,389,799 describes a semiconductor device during whose production the metal for an ohmic contact is implemented after a process of epitaxial growth.
U.S. Pat. No. 5,409,859 describes a method for producing an ohmic contact made of platinum on SiC. There, a doped SiC layer is produced on a p-type SiC single crystal, and a layer of platinum is deposited on that in order to produce the ohmic contact. Annealing (=heating to an elevated temperature) can be effected after implantation of impurity atoms into the SiC layer (post-implant annealing). Annealing of the ohmic contact can additionally be carried out. Whereas the first annealing is carried out before the deposition of the (platinum) metal, the second annealing takes place after the deposition of the (platinum) metal.
It is an object of the present invention, then, to specify a method for producing components having at least one ohmic contact and at least one Schottky contact which overcomes the disadvantages associated with the prior art and in which the production of the ohmic contact does not lead to an impairment of other structures on the component and the ohmic contact, for its part, is insensitive with respect to later method steps at high temperatures.
With the above and other objects in view there is provided, in accordance with the invention, a method for producing a component, which comprises the following method steps:
providing a substrate made from SiC (silicon carbide);
applying a first metal for an ohmic contact on one side of the substrate;
subsequently growing an epitaxial layer on the other side of the substrate at a temperature of more than 1300xc2x0 C.; and
applying a second metal for the Schottky contact on the epitaxial layer at a high temperature.
The invention is based on combining the heat treatment which is repeatedly necessary during the production of the SiC component, i.e. on forming the production of the ohmic contact as early as during the post-implant annealing or during the epitaxy. This results in the greatest possible tightening of the production process.
The novel method for producing a component having a substrate made of SiC, which comprises at least one ohmic contact, which method includes, in addition to the step of applying a first metal layer for the ohmic contact, at least one step in which the substrate is brought to a high temperature, is wherein the first metal layer for the ohmic contact is applied before the last step in which the substrate is brought to a high temperature.
In accordance with an added feature of the invention, the first metal for the ohmic contact is Nb, Ta, Mo, or W.
In particular, it is thus possible to produce a Schottky diode on an SiC substrate by a first metal layer for an ohmic contact being applied on the substrate, an epitaxial layer then being applied on the substrate at a temperature of more than 1300xc2x0 C., and the Schottky contact subsequently being produced by the application of a second metal to the epitaxial layer. These method steps may also be followed by a heat treatment and a cooling process, the application of a contact reinforcing layer on the Schottky contact, patterning of the Schottky metal, application of a contact reinforcing layer to the metal of the ohmic contact on the second side (rear side) of the substrate and also, under certain circumstances, the separation of the substrate into individual chips. In this case, the heating of the substrate during the epitaxy is utilized according to the invention for the production of the ohmic contact. In accordance with an additional feature of the invention, the second metal for the Schottky contact is Ti or Ni.
In a preferred embodiment of the method, the epitaxy and a possible subsequent heat treatment are carried out in a hydrogen atmosphere or in an argon atmosphere.
In order to produce a so-called guard ring on the surface of the component for the purpose of improving the field profile at the edge of the component, the following steps are carried out before the first metal is applied to the rear side of the substrate: growth of an epitaxial layer, production of an implantation mask over the surface of the epitaxial layer, so that an edge region remains free, implantation of impurity atoms in the edge region, thereby producing an implanted edge (guard ring), removal of the implantation mask. This is followed by the application of the rear-side metal and then the heat-treatment step which is necessary for activating the implanted impurity atoms. In other words, the first metal is applied to the rear side of the substrate before the last step of the production method in which the component is brought to a high temperature.
The annealing for activating the implanted ions is preferably carried out at 1400xc2x0 C. to 1700xc2x0 C. for a duration of up to one hour and under an argon or hydrogen atmosphere. The cooling of the component after the annealing is carried out, in particular, under a hydrogen atmosphere.
The implantation can be carried out in such a way as to produce a so-called box profile with an impurity atom concentration which is essentially constant over a predetermined depth below the surface of the substrate.
The separation of the individual chips, referred to as dicing, is preferably effected by sawing the substrate.
The method according to the invention has the advantage that the overall process for producing the component is significantly simplified and accelerated. Furthermore, as a result of the possible Schottky metallization directly after the epitaxy or a possible heat treatment under hydrogen, the quality and yield (reproducibility) are increased in the case of Schottky diodes, and the resulting ohmic rear-side contact is stable up to in excess of 1000xc2x0 C. Consequently, the resulting ohmic rear-side contact is also of interest for the production of components for high-temperature applications, such as e.g. JFETs.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing an ohmic contact, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.